Continuous plating method

ABSTRACT

A method of continuously plating aluminum wire or strip stock. The stock passes through an alkaline etch bath, an acid oxidizing bath, a conditioner bath containing tin, and a bronze-plating bath in which bronze is plated on the aluminum. The conditions of concentration, temperature, and solution movement are all controlled so as to achieve continuous plating of bronze while feeding the stock at a relatively high speed. Following the bronze plating, further plating and/or drawing steps may be carried out.

United States Patent Inventor Lawrence P. Gowman Mariette, Mich.

Appl. No. 826,381

Filed May 21,1969

Patented Nov. 23, 1971 Assignee Wire & Strip Platers, Inc.

Marlette, Mich.

CONTINUOUS PLATING METHOD 3 Claims, 2 Drawing Figs.

Int. Cl C23b 5/50, C23b 5/62 Field of Search 204/27, 33, 44, 49, 40

References Cited UNITED STATES PATENTS 9/1934 Andersen 2,523,161 9/1950Struyk et al 204/49 2,586,099 2/1952 Schultz 204/33 2,650,875 9/1953Dvorkovitz. 204/33 UX 2,891,309 6/1959 Fenster 204/33 3,108,006 10/1963Kenedi et al 204/44 Primary Examiner-John H. Mack AssistantExaminer-W. 1. Solomon *Allomey-Settle, Batchelder and Oltman 1ABSTRACT: A method of continuously plating aluminum wire or strip stock.The stock passes through an alkaline etch bath, an acid oxidizing bath,a conditioner bath containing tin, and a bronze-plating bath in whichbronze is plated on the aluminum. The conditions of concentration,temperature, and solution movement are all controlled so as to achievecontinuous plating of bronze while feeding the stock at a relativelyhigh speed. Following the bronze plating, further plating and/or drawingsteps may be carried out.

CONTINUOUS PLATING METHOD BACKGROUND OF THE INVENTION Although processesor methods for plating metal on alu- 5 minum have been proposedpreviously, to the knowledge of applicant, there has been no processavailable for continuous plating of aluminum wire or strip with bronzeat a relatively high speed; say as high as 50 feet of stock per minuteor even greater. In order to provide such a continuous plating method,the conditions should be based on a long range forecast of consumptionof stock by the user. A speed of wire or strip feed may then be selectedso as to meet the needed production within a minimum space. For example,plating only one strand of eight gauge aluminum wire at 100 feet perminute produces 60 pounds of wire per hour; plating one strand of 14gauge aluminum wire at 100 feet per minute produces 22.5 pounds perhour; and plating one strand of gauge aluminum wire at 100 feet perminute produces 5.6 pounds per hour. Having fixed the production rate,the length and width of the plating line may be fixed, the width beingselected so as to accommodate the number of wires to be plated. Thenumber of baths to be used is selected, and the length of each bath isselected to allow the wire or strip to remain in each bath long enoughto accomplish the required chemical action at the feeding speeds to beused. Since the time for which the stock remains in any one bath islimited, concentration, temperature, and solution movement in the bathare the factors or parameters which can be controlled to controlreaction kinetics. Current density may also be controlled whererelevant. The parameters of batch aluminum plating processes would notapply to a high-speed process such as is needed for economicalcontinuous plating of aluminum wire and strip.

Accordingly, it is an object of the present invention to provide amethod or process for continuous plating of aluminum wire and strip onan economical basis.

Another object of the invention is to provide an aluminumplating processin which parameters such as temperature, concentrations, and solutionmovement are selected and controlled to permit plating of aluminum wireand strip moving at a relatively high speed.

A further object of the invention is to provide a method of continuouslyplating aluminum wire and strip stock in which etching, oxidizing,conditioning, and bronze-plating steps are carried out by passing thestock through successive baths, with any given portion of the stockremaining in each bath only a short time.

Another object of the invention is to condition the aluminum wire orstrip, plate bronze thereon to provide a metallurgical bond between thebronze and the aluminum, and then plate a selected metal on the bronzeto provide a desired coating, suitable metals being tin, copper, lead,lead-tin alloy, or nickel.

Still another object of the invention is to plate metals on aluminumwire as just described in the last preceding paragraph, and then drawthe wire to reduce its size.

A further object is to continuously plate layers of bronze, then nickel,and then tin, lead or lead-tin alloy on aluminum wire or strip toprovide plated material which is solderable and will withstandprescribed solderability tests.

Other objects of this invention will appear in the following descriptionand appended claims, reference being had to the accompanying drawingsforming a part of this specification wherein like reference charactersdesignate corresponding parts in the several views.

ON THE DRAWINGS The two figures of the drawings show a plating line forcontinuous plating of aluminum wire or strip in accordance with theinvention.

GENERAL DESCRIPTION FIGS. 1 and 2 illustrate a processing line forcontinuously plating wire or strip stock in accordance with oneembodiment of the invention. These figures are schematic and are notintended to show the details of processing equipment since suchequipment does not form a part of the present invention.

The stock to be plated is represented in FIGS. 1 and 2 by the line 10which may be wire or strip stock. The stock 10 passes through a seriesof tanks designated 12, l4, l6, 18, 20, 22, and 24. Contact is made tothe stock as by means of rollers or other suitable contact means wherethe stock is to be electroplated. At least one tray for holding a bathis provided in each of these tanks, there being two trays 26 and 28 intank 12, two trays 30 and 32 in tank 14, two trays 34 and 36 in tank 16,one tray 38 in tank 18, one tray 40 in tank 20, one tray 42 in tank 22,and three trays 44, 46, and 48 in tank 24. The stock also passes throughrinse tanks 50, 52, 54, and 56 as shown, and it may be noted that thereis no rinse tank between tanks 16 and 18 nor between tanks 18 and 20.Anodes are used in the electroplating trays, the anode being of themetal being plated. The stock is the cathode in these trays.

The first tank 12 contains an alkaline etching liquid. Some of thisliquid is continuously moved through the trays 26 and 28 by means of apump P, and the liquid overflows from the trays through suitable outletsback to the tank 12. Thus, each of the trays 26 and 28 comprises analkaline etching bath. A suitable composition for the etching liquid is:

Sodium or potassium 3-8 ounces per gallon of hydroxide solution Sodiumor potassium l-4 ounces per gallon of gluconate solution Any suitablewetting agent may also be added to the etching bath, if desired. Thebalance of the bath is water. The temperature of the etching baths 26and 28 is preferably in the range from to F. For high speed plating, thestock remains in each of the baths 26 and 28 a time no greater than 10seconds for each bath, a suitable range being 3 to 10 seconds. Theetching baths 26 and 28 serve to remove oxide from the aluminum stock toexpose the metal.

The term aluminum as used herein is intended to include pure aluminum,commercial grades of aluminum containing normal amounts of impurities,and alloys of aluminum in which aluminum is a major ingredient.

The aluminum stock 10 passes through rinsing bath 50 to remove excessetching liquid and passes from there into baths 30 and 32 in tank 14.

Tank 14 contains an oxidizing liquid which is preferably aqueoushydrochloric or nitric acid containing a fluoride compound. Some of theoxidizing liquid is continuously moved through trays 30 and 32 by meansof a pump P and liquid overflows from trays 30 and 32 through suitableoutlets back to the tank 14. Thus, the trays 30 and 32 contain theoxidizing baths through which the stock 10 passes. The composition ofthe bath may be 50 percent nitric acid or 50 percent hydrochloric acidplus a fluoride salt such as ammonium bifluoride. Hydrofluoric acid canbe used in place of the fluoride salt. Where ammonium bifluoride isused, the concentration is preferably in the range from 4-8 ounces pergallon of solution. The temperature of each of the baths 30 and 32 ismaintained in the range from 70-90 F. The stock 10 remains in each ofthe baths 30 and 32 for a time no greater than 10 seconds where highproduction rates are required. A suitable range for this immersion timeis 3 to 10 seconds.

The baths 30 and 32 serve to reoxidize the aluminum somewhat for thesubsequent treating steps. The stock then passes through the rinsingbath 52 which removes excess oxidizing liquid, and from there passes totrays or baths 34 and 36 in tank 16.

Tank 16 contains a conditioning liquid which conditions the stock for abronze plating step which is to take place in tank 18. Some of thisconditioning liquid is continuously moved through trays 34 and 36 bymeans of a pump P, and the liquid overflows from trays 34 and 36 throughsuitable outlets back to the tank 16. The conditioning ingredient of thebaths 34 and 36 is a stannate compound which is preferably sodiumstannate or potassium stannate. The baths 34 and 36 also preferablycontain sodium or potassium gluconate and sodium or potassium hydroxide.A suitable composition for the baths 34 and 36 is:

Sodium or potassium 12 to l6 ounces per gallon stannate of solutionSodium gluconate l to 3 ounces per gallon of solution Sodium orpotassium l to 3 ounces per gallon hydroxide of solution Balance waterThe baths 34 and 36 should contain at least 5 ounces of tin in the formof a stannate compound per gallon. The tin does not plate out to anysubstantial extent on the aluminum surface, but merely conditions thatsurface to make it receptive to the bronze plating which is carried outin tanks 18 and 20. The sodium or potassium gluconate ingredient of thebaths 34 and 36 is not believed to be essential, but is helpful to keepthe aluminum surface clean by chelating alloying constituents. Thetemperature of the baths 34 and 36 is preferably maintained in the rangefrom 80 to 100 F. The stock 10 remains in the baths 34 and 36 a time nogreater than l seconds for high production rates, and a suitable rangefor this immersion time is l to 10 seconds.

The stock 10 passes from tank 16 without any intervening rinsing bath tothe baths 38 and 40 of tanks 18 and 20 respectively. Tanks l8 and 20each contain a bronze-plating solution. This is an aqueous solutioncomprising in ounces per gallon of solution (a) at least ounces of tincontained in a stannate compound, (b) at least 2.5 ounces of coppercontained in a cyanide compound, and (c) at least 1 ounce of alkalihydroxide. The solution is moved through trays 38 and 40 by means ofpumps P and the solution is allowed to overflow from the trays 38 and 40through suitable outlets back to the tanks 18 and 20.

A suitable composition for the bronze plating solution is:

Sodium or potassium 12-16 ounces per gallon stantiate Sodium orpotassium 3-49; ounces per gallon cyanide ZKCNCuCN l2l 5 ounces pergallon The balance of the solution is water.

The solution is preferably maintained in a temperature range from 90 tol20 F. A cathode current density of 100-600 amperes per square foot ismaintained. The stock remains in each of the baths 38 and 40 for a timeno greater than seconds for high production rates, and a suitable rangefor this immersion time is 3 to 10 seconds.

These high current densities are required to plate a suitable thicknessof bronze on the aluminum stock. A suitable thickness is 0.000030 inch.

The bronze plate should have good adherence to the aluminum, and goodadherent platings of bronze have been achieved with the processingconditions outlined above. The bronze plate provides a metallurgicalbond to the aluminum, and makes it possible to plate other metals on thestock in subsequent processing steps. An example of such processingsteps is illustrated by the tank 24 in the drawing. One of the mostdesirable materials to plate on the wire in the tank 24 would be asolderable metal or alloy such as tin or lead-tin alloy. However, it hasbeen found that in order for the tin or lead-tin material to be bondedin a manner such that the resulting product will meet the solderabilitytest known as a solder pot test, it has been found to be desirable toplate a barrier layer of nickel on the bronze plated stock before thetin or lead-tin material is plated in tank 24. Therefore, the processingline illustrated in the drawing also includes a nickel plating tank 22having a tray 42 therein. It is to be understood that for someapplications it would not be essential to use the nickel plating tank22. Thus, it is possible to plate a metal such as tin, copper, lead-tin,lead or nickel on the bronze-plated wire without any intervening barrierif there is no need to pass a solderability test such as the solder pottest. Where a nickel barrier layer is used, it serves to prevent thebronze from dissolving in the metal or alloy plated in tank 24 and thusimpairing the metallurgical bond.

Where a nickel-plating bath is used, the solution is preferably anaqueous solution of nickel fluoborate. A suitable composition for thisbath is two parts of nickel fluoborate to one part of water. The pH ofthe solution is preferably maintained at about 3 to 4%. The stock is thecathode, and a cathode current density of to 500 amperes per square footis maintained. The temperature of the nickel-plating bath is preferablymaintained in the range from about 100 to F. The stock passes throughthe nickel-plating bath in a time no greater than 10 seconds for highvolume production.

From the nickel plating bath 42, the stock passes through a rinsing bath56 to the baths 44, 46, and 48 provided in the plating tank 24. Aspreviously mentioned, any of several metals or alloys may be plated onthe stock in the tank 24, and no attempt will be made to illustrate allof the plating solutions which are possible. Preferably, the selectedmetal should be plated from a fluoboric solution of the metal.

It is to be noted that in the foregoing description, the stock passesthrough each of the baths in a time of 10 seconds or less. For someapplications, it may be possible to increase this time, but in generalthe parameters of the process are selected such that successful platingcannot be achieved at extremely slow speeds of the stock.

EXAMPLE 1 The stock in this example was 0.040 inch thick by 2 incheswide aluminum strip of the type known as alloy No. 1100-0. The speed ofthe stock was 50 ft./min. The stock passed through trays 26 and 28containing the etching liquid in a time of IO seconds, and thetemperature of these baths was maintained at F. The composition of thesebaths was in accordance with the previous description. The stock passedthrough each of the oxidizing baths 30 and 32 in a time of 10 secondsfor each bath, and the temperature of these baths was maintained atabout 75 F. The composition of the oxidizing bath 30 and 32 and otherprocessing conditions were in accordance with the previous description.

The stock passed through baths 34 and 36 in a time of 10 seconds each,these baths being maintained at a temperature of about 95 F. The baths34 and 36 contained about 5.5 ounces of tin metal in the form ofpotassium stannate per gallon of solution.

The stock passed through each of the bronze-plating baths 38 and 40 in atime of 10 seconds, and a current density of 270 a./square foot was usedas the cathode current density. The temperature of the baths 38 and 40was maintained at about 100 F. The baths 38 and 40 contained about 3oz./gal. of copper in the form of copper cyanide, about 4 oz./gal. offree cyanide, about 5.5 oz./gal. of tin in the form of potassiumstannate, and about 1.5 oz./gal. of free caustic in the form ofpotassium hydroxide. A nickel plating bath 22 was not used in thisexample, but rather the material was passed directly into tank 24 wheretin was plated on the stock. A tin metal concentration of about 1 loz.lgal. was used at a temperature of 90 F. and a cathode currentdensity of a./square foot.

With these processing conditions, the resulting product was adherent,and could be soldered satisfactorily by normal soft solderingtechniques.

EXAMPLE 2 In this example, eight gauge aluminum wire was passed throughthe processing line at a speed of 100 ft./min. The wire passed throughthe etching baths 26 and 28 in a time of 10 seconds for each bath. Thetemperature of the etching baths was maintained at about 155 F. Thecomposition of the etching baths was the same as in example 1.

The wire passed through the oxidizing baths 30 and 32 in about secondsfor each bath, and the temperature of these baths was maintained atabout 75 F. The oxidizing baths contained 50 percent by volume nitricacid and 4 oz./gal. of ammonium bifluoride. The balance of the solutionwas water.

The stock passed through the conditioner baths 34 and 36 in a time of 5seconds for each bath, and the temperature of these baths was maintainedat 90 F. These baths contained about 6.5 oz./gal. of solution of tinmetal in the form of potassium stannate.

The wire passed through the bronze plating baths 38 and 40 in a time of10 seconds for each bath, and the baths were maintained at a temperatureof 95 F. A cathode current density of 540 a.lsquare foot was used. Thecomposition of the baths was the same as in example 1. Thenickel-plating bath 42 was not used.

The wire passed through baths 44, 46, and 48 in a time of 5 seconds each(total immersion time seconds), and the temperature of the platingsolution was 85 F. A cathode current density of 200 a./square foot wasused. The material plated on the wire was tin-lead alloy, and this alloywas plated from a fluoboric solution of standard composition.

The resulting product had an adherent plating, and good solderablecoatings were obtained.

EXAMPLE 3 In this example, gauge aluminum wire alloy identified as alloyNo. 1245 was passed through the processing line at a speed of 200ft./min. The stock passed through the etching baths 26 and 28 in a timeof 5 seconds for each bath. The temperature of the baths was maintainedat about 170 F. The compositions of the baths were in accordance withthe previous description.

The wire passed through the oxidizing baths 30 and 32 in a time of 5seconds for each bath, and the other processing conditions were the sameas for example 1.

The wire passed through the conditioning baths 34 and 36 in a time of 5seconds for each bath, the temperature of these baths was maintained at100 F. The other processing conditions were the same as for example 1.

The stock passed through the bronze plating baths 38 and 40 in a time of5 seconds for each bath, and the temperature of these baths wasmaintained at about 110 F. A cathode current density of 1,080 a./squarefoot was used. The composition of the baths was about 3.5 oz./gal. ofcopper metal, about 6.5 oz./gal. of tin metal, about 4.5 oz./gal. offree cyanide in the form of potassium cyanide and about 1.7 oz./gal. offree caustic in the form of potassium hydroxide. The balance of thesolution was water.

The stock passed through the nickel plating bath 42 in a time of 2.5seconds, and the bath was maintained at 115 F. A cathode current densityof 300 a./square foot was used. The composition of the nickel-platingbath was in accordance with the previous description.

The wire passed through the tin-plating baths 44, 46, and 48 in a timeof 7.5 seconds, and the temperature of these baths was about 100 F. Thecathode current density was 200 a.lsq. foot. The metal plated on thewire in the baths 44, 46, and 48 was tin, and a standard tin fluoborateplating solution was used.

The resulting product had highly adherent coatings, and coatings wereobtained which were easily solderable by normal soft solderingtechniques. These coatings also withstood a solder pot test which ismore rigid than normal soft soldering techniques.

After plating of the metal on the stock, it is possible to draw thestock to reduce its size. In experimental runs, a reduction of fourgauge sizes after the plating process has been carried out has beensuccessfully achieved.

To determine whether good solderable coatings have been obtained,certain tests can be carried out. When a nickel barrier layer is notused, a coating of tin or lead-tin alloy can be provided onbronze-plated aluminum stock which is solderable by normal softsoldering techniques. The solderability can be checked by soldering ajoint between two pieces of stock using a solder such as 6040 lead-tinalloy at a temperature of 240350 F. The joint is then stressed to thebreaking point, and if a break occurs in the solder rather than betweensolder and aluminum, the solder joint was sound and the coatings aresatisfactory.

Where soldering is to be carried out at high temperature or withprolonged exposure to heat, a test known as the solder pot test is used.This test is described in U.S. military specification MIHTD 202 C datedSept. 12, 1963. In general, soldering is carried out at a temperature inthe range from 350 to 500 F. (say 450 F.) for 10 seconds, and theresults are satisfactory if good wetting of the solder on the stock isachieved.

Having thus described my invention, 1 claim:

1. A method of continuously plating the surfaces of aluminum wire orstrip stock, comprising the steps of:

1. feeding the aluminum wire or strip stock to a series of baths,

2. passing said stock through an aqueous alkaline etch bath containingalkali hydroxide in sufficient concentration and temperature to stripsubstantially all of the oxide from the aluminum surfaces of said stock,

3. passing said stock through an aqueous rinse bath to remove the alkalihydroxide from the stock,

4. passing said stock through an aqueous acid bath containing nitric orhydrochloric acid and a fluoride compound in sufficient concentrationsto provide an oxide layer on the aluminum surfaces of said stock andremove foreign matter,

5. passing said stock through an aqueous rinse bath to remove the acidfrom the stock,

6. passing said stock through an aqueous conditioning bath containingsodium or potassium stannate in sufiicient concentration to conditionthe aluminum surfaces of the stock, which is substantially about 5ounces of tin per gallon of solution contained in a stannate compound,and sufficient concentration of alkali hydroxide to remove the oxidefrom the aluminum surfaces of the stock sufficiently to permitsubsequent bronze plating and substantially prevent immersion plating ofthe tin from the stannate on the aluminum surfaces, and,

7. passing said stock as a cathode directly into an aqueousbronze-plating bath, without rinsing, containing an alkali hydroxide insubstantially the same concentrations as in said conditioning bath, andsufficient concentrations of tin in a stannate compound and copper in acyanide compound to plate bronze on the conditioned aluminum surface ofsaid stock while maintaining a cathode current density of less than 500amperes per square foot.

2. The plating method as claimed in claim 1 including the further stepof passing said stock as cathode through a nickelplating bath comprisedof an aqueous solution of nickel fluoborate while maintaining in saidnickel-plating bath a cathode current density sufficient to plate nickeland less than 500 amperes per square foot.

3. The plating method as claimed in claim 2 including the further stepof passing said stock as cathode through a further plating bathcomprised of a fluoboric solution of a material selected from the groupconsisting of tin, copper, lead-tin, lead, and nickel to plate theselected material on said stock.

i i i

2. passing said stock through an aqueous alkaline etch bath containingalkali hydroxide in sufficient concentration and temperature to stripsubstantially all of the oxide from the aluminum surfaces of said stock,2. The plating method as claimed in claim 1 including the further stepof passing said stock as cathode through a nickel-plating bath comprisedof an aqueous solution of nickel fluoborate while maintaining in saidnickel-plating bath a cathode current density sufficient to plate nickeland less than 500 amperes per square foot.
 3. The plating method asclaimed in claim 2 including the further step of passing said stock ascathode through a further plating bath comprised of a fluoboric solutionof a material selected from the group consisting of tin, copper,lead-tin, lead, and nickel to plate the selected material on said stock.3. passing said stock through an aqueous rinse bath to remove the alkalihydroxide from the stock,
 4. passing said stock through an aqueous acidbath containing nitric or hydrochloric acid and a fluoride compound insufficient concentrations to provide an oxide layer on the aluminumsurfaces of said stock and remove foreign matter,
 5. passing said stockthrough an aqueous rinse bath to remove the acid from the stock, 6.passing said stock through an aqueous conditioning bath containingsodium or potassium stannate in sufficient concentration to conditionthe aluminum surfaces of the stock, which is substantially about 5ounces of tin per gallon of solution contained in a stannate compound,and sufficient concentration of alkali hydroxide to remove the oxidefrom the aluminum surfaces of the stOck sufficiently to permitsubsequent bronze plating and substantially prevent immersion plating ofthe tin from the stannate on the aluminum surfaces, and,
 7. passing saidstock as a cathode directly into an aqueous bronze-plating bath, withoutrinsing, containing an alkali hydroxide in substantially the sameconcentrations as in said conditioning bath, and sufficientconcentrations of tin in a stannate compound and copper in a cyanidecompound to plate bronze on the conditioned aluminum surface of saidstock while maintaining a cathode current density of less than 500amperes per square foot.